Routers are commonly used to forward data packets among subnets of a Layer-3 network. (A subnet, short for subnetwork, is a logical subdivision of a Layer-3 network.) Network ports of nodes within a given subnet share the same Layer-3 network address prefix. For example, in Internet Protocol (IP) networks, the ports in each subnet share the same most-significant bit-group in their IP address, so that the IP address is logically divided into two fields: a network or routing prefix, and the rest field or host identifier. Similarly, in InfiniBand™ (IB) networks, each subnet is uniquely identified with a subnet identifier known as the Subnet Prefix. For each port in the subnet, this prefix is combined with a respective Globally-Unique Identifier (GUID) to give the IB Layer-3 address of the port, known as the Global Identifier (GID).
Typically, the logical subdivision of a Layer-3 network into subnets reflects the underlying physical division of the network into Layer-2 local area networks. Routers forward packets among subnets on the basis of their Layer-3 (IP or GID) destination addresses, while within a given subnet packets are forwarded among ports by Layer-2 switches or bridges. These Layer-2 devices operate in accordance with the applicable Layer-2 protocol and forward packets within the subnet according to the Layer-2 destination address, such as the Ethernet™ medium access control (MAC) address or the IB link-layer Local Identifier (LID). In general, Layer-2 addresses in a given subnet are recognized only within that subnet, and routers will swap the Layer-2 address information of packets that they forward from one subnet to another.
Routers in most existing networks—particularly IP networks—use the Address Resolution Protocol (ARP) to resolve Layer-3 addresses of packets that they receive into Layer-2 addresses that can be used for local forwarding within the destination subnet. ARP was first defined by Plummer in “An Ethernet Address Resolution Protocol,” published as Request for Comments (RFC) 826 of the Internet Engineer Task Force (IETF), in 1982. ARP is a request/reply protocol that specifies a packet structure that can be used by nodes in a network to query and inform one another as to the Ethernet address that corresponds to any given IP address. Routers use ARP to build lookup tables of the correspondence between IP and Layer-2 addresses, which they can then use in packet forwarding.
In some multicast applications, MAC addresses may be defined on the basis of corresponding IP multicast addresses. A scheme of this sort is defined, for example, by Hinden et al., in “IP Version 6 Addressing Architecture,” published as IETF RFC 2373 (1998). In this scheme, IPv6 multicast addresses are mapped into Ethernet MAC addresses by taking the low-order 32 bits of the IPv6 multicast address and using them to create a MAC multicast address. Thus, new IPv6 multicast addresses are assigned so that the group identifier is always in the low-order 32 bits.